1. Technical Field
This invention relates to chemical-mechanical planarization (CMP) of sapphire, and, more particularly, to a slurry for chemical-mechanical planarization of sapphire and to a method for preparing the same.
2. Background of the Invention
Single crystal sapphire (α-Al2O3) possesses a unique combination of excellent optical, physical and chemical properties. The hardest of the oxide crystals—measuring 9 on the Mohs' scale of hardness—sapphire retains its high strength at high temperatures, has good thermal and electrical properties and excellent transparency. It is chemically resistant to common acids and alkali at temperatures of up to 1000° C. However, it is difficult to process by processes such as etching and device separation using mechanical cutting or scribing due to its high chemical and thermal stability, and hardness.
The excellent optical, physical and chemical properties of sapphire encourage its wide use in hostile environments where optical transmission in the range from the vacuum ultraviolet to the near infrared is required. Accordingly, sapphire is the material of choice in a variety of modern hi-tech applications, including space and military optical systems, high-power laser optics, and high-pressure components. Sapphire is an anisotropic hexagonal crystal, and its properties depend on the crystallographic direction (relative to the optical C-axis).
Gallium nitride (GaN) is regarded as the third generation semiconductor material, after silicon (Si) and gallium arsenide (GaAs), and it has been used in semiconductor components since the 1990's. However, it is difficult to form GaN into crystalline material, and to be a useful semiconductor, it must be grown as a film on other substrates, such as, e.g., on Al2O3, SiC, Si, MgO, or ZnO. Sapphire is the foremost substrate material for crystallizing GaN. At present, GaN can be epitaxied on a sapphire substrate to produce GaN-based blue LEDs and LDs.
It is well known that device quality depends to a great extent on the surface machining quality of the substrate. Precision finishing technology for sapphire substrate for GaN growth is an especially difficult problem, and much of present research focuses thereon. Although a super-smooth crystal surface is required, sapphire is difficult to machine process because of its high hardness and brittleness. With the rapid development of optoelectronic technology, the demand for sapphire substrate material for optoelectronic products has increased significantly. In order to meet the demand, the mechanism, technology, and the selection of materials for sapphire polishing has become a very important issue.
Conventional technology for batch machining sapphire is not yet sufficiently well developed. When producing sapphire wafers, the percentage of wafers with cracks and edge damage is quite high and ranges form 5 to 8%. In addition, twenty percent of processed wafers require reworking due to serious surface scratches caused by polishing and grinding steps. Because partial rework wafers can't be used in many applications, this leads to an increased material cost. During the grinding and polishing steps, the polishing and grinding rate is also very low, (1-5 μm/h).
Conventional slurries used for chemical-mechanical planarization of sapphire consist mostly of Al2O3, which has high hardness and strong mechanical action. However, using Al2O3 often produces undesirable scratches and other defects. Accordingly, there is much room for improvement in the area of chemical-mechanical polishing of sapphire as related to surface smoothness, processing speed, and cost.